This invention relates to backlit liquid crystal display devices (LCDs) having a first light source for operation during the day, and a separate and independent second light source for operation at night.
LCDs are gaining in popularity for use in systems such as television receivers, computer monitors, avionic displays, aerospace displays, and other military-related displays, where the elimination of cathode ray tube technology is desirable for several reasons. In particular, cathode ray tubes are characterized by large depth dimensions, inordinately high weight and extreme fragility. Additionally, cathode ray tubes require a relatively high voltage power supply in order to sufficiently accelerate the electron beam, and thus sustain the displayed image.
The aforementioned shortcomings of cathode ray tubes (CRTs) are overcome by the flat panel liquid crystal display in which a matrix array of liquid crystal picture elements or pixels are arranged in a plurality of rows and columns. Patterns of information are thereby defined by the two-dimensional array of pixels which, because of differences in the orientation of the liquid crystal material within each pixel, are caused to appear either darkened or transparent.
Liquid crystal displays may be either transflective or transmissive. Transflective displays depend upon ambient light conditions in order to be viewed, i.e. light from the surrounding environment, incident upon the side of the display facing the viewer, is reflected back to the viewer. Transflective liquid crystal displays cannot, therefore, be used in a dark or low light environment, since there is no light available for reflection off the viewing surface of the display.
Conversely, transmissive liquid crystal displays require the use of illuminating means, such as a tubular fluorescent lamp array operatively disposed on the side of the matrix array of pixels opposite the viewer. This illumination means, or backlight, may also include a back reflector adapted to efficiently redirect any stray illumination towards the matrix array of rows and columns of picture elements, thus ensuring that the displayed image is as bright as possible (given the characteristics of the lighting scheme employed).
In the past, a great deal of research in the field of flat panel liquid crystal display devices has been dedicated to the design of backlighting schemes which optimize viewing and structural parameters of those displays. Particularly, uniformity and intensity of light across the illuminated area has been maximized while maintaining low power consumption and a low overall profile, i.e., a thin assembly.
For example, as disclosed in the commonly assigned U.S. Pat. No. 5,161,041, the entire disclosure of which is incorporated herein by reference, integral image splitting and collimating means operatively disposed between the light source and the rows and columns of liquid crystal picture elements was employed. This integral image splitting and collimating means has the advantages of providing a bright, uniform light to the matrix array of pixels, while maintaining a narrow profile and minimizing the power consumption of prior backlit electronic displays. This bright, uniform light achieves a high contrast display in bright ambient light conditions.
The effect of the integral image-splitting and collimating means is to eliminate local bright spots and pale spots in the display, corresponding, respectively, to the legs and the spaces between the legs of a typical fluorescent lamp, by providing two similar images of the light emanating from each lamp leg. By locating the split images contiguous, one to each other, the area of illumination is effectively enlarged, and a bright, uniform light distribution across a low profile LCD is obtained. In addition to image-splitting, the specific integral image-splitting and collimating means employed in the aforementioned patent provided collimated light. Additionally, when a light diffuser is provided between the integral image-splitter/collimator and the matrix array, wide angle viewability is also achieved. The precise diffuser chosen depends on the application of the LCD.
In preferred forms of the invention U.S. Pat. No. 5,161,041, the integral collimating and image-splitting means included a thin film having light-refracting, faceted prisms formed on one of its faces. An example of such a film is 3M SCOTCH.TM. Optical Lighting Film. In preferred forms, this thin 3M SCOTCH.TM. film is used by laminating it to a clear transparent sheet of glass, ceramic or plastic, and thereafter using it as a layer in a low profile LCD stack.
While the above-mentioned U.S. patent improved the profile and optical characteristics of prior art electronic displays, and also improved lighting efficiencies so as to reduce the power consumption of the displays, that application did not deal with the problem associated with the use of such displays at nighttime, when very low light intensities are desirable. In particular, known devices utilize tubular fluorescent lamps to provide the high intensity light required for high contrast color liquid crystal displays used during daylight operation where high ambient light conditions exist, e.g., for avionic applications. However, when dimmed to the low intensity levels required for nighttime use, fluorescent lamps loose stability and uniformity. Loss of stability is used herein to mean that the fluorescent lamps begin to flicker. Loss of uniformity is used herein to mean that light and dark bands appear along the fluorescent lamp.
Other research regarding the use of LCDs has concentrated on their use when the viewer is wearing night vision goggles (NVG). NVGs are designed to detect infrared light and are used typically in very low levels of light. The major problem associated with the use of NVGs occurs when stray light, and particularly stray infrared light, is reflected into the NVG, saturating it. The stray light often comes from displays and panel equipment and reflections of the light therefrom.
Night vision goggles operate because of their high sensitivity to very low levels of light, mainly in the near infrared (IR) region of the spectrum (i.e. about 630-1100 nm). Efforts to block the IR region of the displays and panel equipment were unsuccessful because color integrity (particularly of the color red) and the ability to view the LCD at reasonably wide angles from normal (e.g. up to about 60.degree.) could not be achieved. The new sharp cutoff IR filter of my commonly-assigned, co-pending U.S. patent application Ser. No. 925,193, (filed Aug. 6, 1992 and entitled "Night Vision Goggle Compatible Liquid Crystal Display Device", the disclosure of which is incorporated herein by reference) provided a solution to these problems. This new filter is successful because, while it does cut off the IR region of the spectrum, it does not cut off a portion of the visible red light resulting in an unbalanced white color, and a shifting of the red color towards the orange. The resulting display thus can pass the NVIS-B criteria of Military Standard MIL-L-85762A. Additionally, by combining the new sharp cutoff IR filter of U.S. patent application Ser. No. 925,193 with the integral image-splitting and collimating means of U.S. Pat. No. 5,161,041, reasonably wide viewing angles may also be achieved.
U.S. Pat. No. 5,143,433 discloses a night vision compatible backlighting system for a liquid crystal display, including a first group of light sources for activation during the day and a second group of light sources for activation at night. In the preferred embodiments of this patent the nighttime light sources are positioned directly behind the daytime light sources whereby the daytime light sources are disposed directly between the display panel and the nighttime sources. Therefore, the light from the nighttime light sources must pass through the daytime lamps, being diffused thereby, before reaching the active matrix display panel.
U.S. Pat. No. 5,143,433 further states that if the nighttime lamps are arrayed and located not to reside directly behind the existing daytime lamp tubes, the use of a diffusing plate can be used as may be necessary. In such a case, the diffusion plate may be located either at the liquid crystal display or just above the low level intensity nighttime lamps.
A problem associated with the daytime/nighttime liquid crystal display backlighting system of U.S. Pat. No. 5,143,433 is that the daytime bulbs or lamps often have characteristics (i.e. thickness, coatings, etc.) which prohibit them from satisfactorily diffusing the light emitted from the nighttime sources, thus, resulting in erratically diffused low intensity light at nighttime giving rise to local bright spots and pale spots in the display. Furthermore, U.S. Pat. No. 5,143,433 while discussing the location of the nighttime light sources not directly behind the daytime sources, does not illustrate or disclose how such a system could be arranged or practiced. Also, the device of the aforesaid patent, while providing separate daytime and nighttime light sources, requires an increased depth or profile of the display due to the presence of both the separate backlights and the daytime reflector.
While the aforesaid prior art teaches the use of a liquid crystal display backlighting system including both nighttime and daytime light sources, the prior art does not teach or suggest how to practice such an invention such that the light emitted from both the daytime and nighttime light sources is split into two similar images thereof which are cojoined to create a substantially uniform generation of light for impingement upon the rear side of the liquid crystal matrix array while maintaining a display with a low profile or depth.
As will be discussed more fully below, the instant invention provides solutions to the above-described problems of the prior art, and improves the compatibility of LCDs with NVGs.